Illumination device and method for avoiding an over-power or over-current condition in a power converter

Abstract

An illumination device and methods are provided herein for avoiding over-current and over-power conditions in one or more power converters included within the illumination device. The illumination device may include at least a plurality of light emitting diode (led) chains, a driver circuit, at least one power converter, and a control circuit. In some embodiments, the control circuit may be generally configured for determining a maximum safe current level and/or a maximum safe power level attributed to the power converter(s) at a present operating temperature, and for adjusting respective drive currents supplied to the plurality of led chains by the driver circuit, so as not to exceed the maximum safe current level or the maximum safe power level at the present operating temperature. In some embodiments, a temperature sensor may be included within the illumination device for measuring the operating temperature presently associated with the power converter(s).

Description

where Y₁, Y₂, Y₃, and Y₄ represent the lumen proportions of the four emission LED chains. These lumen proportions (Y₁, Y₂, Y₃ and Y₄) may be calculated using well-known color mixing equations, the Target Chromaticity (xm, ym) values set within the illumination device, and the expected color points (x₁, y₁), (x₂, y₂), (x₃, y₃), (x₄, y₄) determined in step 74 of FIG. 7B. As these equations are well-known and readily understood by a skilled artisan, further description of such equations will be omitted herein. In one example, the lumen proportions determined in step 76 may be 0.2, 0.2, 0.2 and 0.4 for chains of red, green, blue and white LEDs, respectively. Of course, substantially different lumen proportions may be determined for different Target Chromaticity values and different combinations of LED chains.

Once the lumen proportions (e.g., Y₁, Y₂, Y₃, and Y₄) are calculated for each emission LED chain in step 76 of FIG. 7B, the method calculates the Relative Lumens needed from each LED chain at 25° C. to achieve the lumen proportions in step 78 of FIG. 7A. In calculating the Relative Lumens, one of the LED chains is assumed to be driven with a maximum drive current to produce a maximum lumen output, as described above. For example, the method may assume that a chain of white LEDs is driven with a maximum drive current (e.g., 900 mA) to produce a maximum lumen output of, e.g., 1000 lumens. If the lumen proportions determined in step 76 are 0.2, 0.2, 0.2 and 0.4 for chains of red, green, blue and white LEDs, the Relative Lumens needed from each LED chain to achieve the lumen proportions would be 500 lumens from the red LED chain, 500 lumens from the green LED chain, 500 lumens from the blue LED chain, and 1000 lumens from the white LED chain.

In step 80, the Relative Lumens from step 78 are divided by the maximum lumens that can be produced by each LED chain at 25° C. (which is known and stored in memory as discussed above) to determine a ratio of Relative Lumens over maximum lumens for each LED chain. In the above example, a ratio of Relative Lumens over maximum lumens may be:

500/250=2 for the red LED chain;

500/400=1.25 for the green LED chain;

500/50=10 for the blue LED chain; and

1000/1000=1 for the white LED chain.

In step 82, the Actual Lumens needed from each LED chain to achieve the Target Chromaticity at 25° C. is determined by dividing the Relative Lumens from step 78 by the largest ratio calculated in step 80. In the above example, the LED chain with the largest ratio (e.g., 10) is the blue LED chain. Thus, the Actual Lumens may be determined in the current example by dividing the Relative Lumens (e.g., 500, 500, 500 and 1000 lumens) determined in step 78 for the red, green, blue and white LED chains by 10 to achieve an Actual Lumens of 50 lumens from the red LED chain, 50 lumens from the green LED chain, 50 lumens from the blue LED chain, and 100 lumens from the white LED chain.

In step 84, the Actual Lumens from all LED chains are summed to determine the Max Lumens that can be produced by all LED chains at 25° C. In the current example, a Max Lumens of 50+50+50+100=250 lumens is determined (in step 84) and temporarily stored in memory (in step 86). Once the Max Lumens value is determined, process flow returns to step 56 of FIG. 6.

Step 56 of FIG. 6 determines the Drive Currents (Ix) that are needed from each LED chain to produce the Actual Lumens for each chain at the predetermined safe temperature. According to one embodiment, the Drive Currents (Ix) may be determined using a forward voltage (Vfe_safe), which was previously calibrated for each LED chain at the predetermined safe temperature, the Actual Lumens values determined in step 82 for each LED chain, the table of calibration values stored within the illumination device, and one or more interpolation techniques.

The graph shown in FIG. 10 depicts how one or more interpolation technique(s) may be used to determine the Drive Currents (Ix) needed from each LED chain (in step 56) to produce the Actual Lumens (Lx) determined in step 82 of FIG. 7A. In FIG. 10, the solid dots (●) represent exemplary luminous flux calibration values, which were previously obtained during calibration of the illumination device at three different drive currents (e.g., 10%, 30% and 100% of the maximum drive current) and two different temperatures (e.g., T0 and T1) and stored within the table of calibration values. Exemplary methods for obtaining such calibration values are described in co-pending application Ser. Nos. 14/314,451 and 14/471,057, which are incorporated herein in their entirety. The stored calibration values are not limited to only those shown in FIG. 10.

In some embodiments, two interpolation techniques may be needed to determine the Drive Currents (Ix) that are respectively needed for each LED chain to produce the Actual Lumens (Lx) determined in step 82. For example, a first linear interpolation may be applied to the stored luminous flux calibration values (●) to calculate the luminous flux values (Δ), which should be produced at the predetermined safe temperature (Vfe_safe) when using the same three drive currents (e.g., 10%, 30%, and 100% of the maximum drive current) used during the calibration phase. If the Actual Lumens (Lx) produced by a given LED chain differs from one of the calculated luminous flux values (Δ), a second interpolation may be applied to the calculated luminous flux values to generate a relationship there between (denoted by the solid line in FIG. 10). The second interpolation may be linear or non-linear depending on the color of the LED chain. From this relationship, the Drive Currents (Ix) needed for a given LED chain to produce the Actual Lumens (Lx) may be determined.

In other embodiments, only one interpolation technique may be used to determine the Drive Currents (Ix) that are needed for each LED chain to produce the Actual Lumens (Lx) determined in step 82. For example, if the luminous flux calibration values (●) were previously measured at the predetermined safe temperature (i.e., if T0=25° C.), a linear or non-linear interpolation technique may be applied directly to the stored luminous flux calibration values (●) to determine a relationship there between (denoted by the dashed line at Vfe @ T0 in FIG. 10). From this relationship, the Drive Currents (Ix) needed for a given LED chain to produce the Actual Lumens (Lx) may be determined.

Once the Drive Currents are known, the total power (“Total Power”) drawn by all LED chains at the predetermined safe temperature may be estimated (in step 58). The Total Power drawn by all LED chains is the sum of the power drawn by each individual chain (e.g., P1+P2+P3+P4 when four LED chains are included). In one embodiment, the power drawn by each individual LED chain can be estimated by multiplying a respective Drive Current (Ix) with a forward voltage value (Vfe_est) estimated for that Drive Current at 25° C. In one example, the forward voltage values (Vfe_safe) that were previously calibrated for each LED chain at 25° C. may be scaled (e.g., by some fixed amount or by using characterization data and a curve fitting approach) to estimate the forward voltage values (Vfe_est) corresponding to the Drive Currents.

In step 60, a Scale Factor is generated for adjusting a Target Lumens value set for the illumination device to ensure that the Drive Currents determined for each LED chain (in step 56) and the estimated Total Power drawn by all LED chains (in step 58) at the predetermined safe temperature will not exceed a maximum safe current level (“Max Current”) or a maximum safe power level (“Max Power”) attributed to the power converters (e.g., power converters 14 and 32 of FIGS. 1-2) of the illumination device at the predetermined safe temperature.

FIG. 11 is a flowchart diagram illustrating one embodiment of a method, which can be used to generate a Scale Factor for a predetermined safe temperature. As shown in FIG. 11, the Scale Factor value may be temporarily set to “1” (in step 90) for a first iteration of possibly multiple iterations used to generate the Scale Factor value. In this embodiment, the maximum safe current level (“Max Current”) and the maximum safe power level (“Max Power”) attributed to the power converter(s) may then be determined for the predetermined safe temperature (in step 94).

As shown in FIG. 3, the saturation current (I_sat) associated with a power converter decreases linearly with increasing temperatures above the predetermined safe temperature (e.g., 25° C.), and in some cases, may decrease as much as 30-40% over a 25° C.-100° C. temperature range. This decreasing saturation current decreases the Max Current associated with the DC/DC converters 32 and the Max Power associated with the AC/DC converter 14. In some embodiments, a relationship of I_sat vs. temperature may be stored within a storage medium of the illumination device (e.g., storage medium 23) for each power converter. In one example, the slope and intercept of the I_sat vs. temperature relationship shown in FIG. 3 may be stored for each power converter. While the relationship of I_sat vs. temperature may be similar for each power converter, slightly different slope and intercept values may be stored so that each power converter can be individually characterized.

From the stored I_sat vs. temperature relationships, the Max Current associated with each of the DC/DC converters 32 and the Max Power associated with the AC/DC converter 14 may be determined at the predetermined safe temperature by linearly interpolating between the stored values (in step 94). In one embodiment, the Max Current at 25° C. may be approximately 900 mA for the white, red and green LED chains and approximately 400 mA for the blue LED chain, and the Max Power at 25° C. may be approximately 16 W.

In step 96, a ratio of Max Power (from step 94) over Estimated Total Power (from step 58) is calculated for the AC/DC converter 14. In step 98, a ratio of Max Current (from step 94) over Drive Current for each LED chain (from step 56) is calculated for each of the DC/DC converters 32. The smallest of the ratios calculated in steps 96 and 98 is multiplied with the Scale Factor value (e.g., “1” from step 90 if on first iteration) and the result is stored as a new Scale Factor value (in step 100). If the result is greater than 1, the new Scale Factor value is clipped at 1.

As noted above, the Drive Currents (Ix) determined in step 56 of FIG. 6 were calculated under the assumption that one LED chain was driven with a maximum drive current to provide a maximum lumen output (step 78 of FIG. 7A). This means that one of the Max Current/Drive Current ratios determined in step 98 will be “1” (for the LED chain driven with maximum drive current), and the Max Current/Drive Current ratios for the other LED chains should be values less than or equal to “1.” The Max Power/Total Power ratio may be more or less than one, depending on the combined Drive Currents (step 56) needed to achieve the Target Chromaticity. The smallest of the ratios calculated in steps 96 and 98 is used in step 100 to generate the Scale Factor. In one example, a Scale Factor of 0.5 may be generated in step 100 if the estimated Total Power is twice as much as the Max Power.

Once the Scale Factor is determined (in step 100), a Target Lumens value is calculated (in step 62 of FIG. 6) according to the equation:
Target Lumens=Brightness*Max Lumens*Scale Factor
where “Brightness” typically refers to the brightness setting stored within the illumination device, “Max Lumens” refers to the Max Lumens value calculated in step 54, and “Scale Factor” refers to the scale factor generated in step 100. In this step, however, the Target Lumens value is calculated with the Brightness value temporarily set to “1,” and the results of the calculation are used to update the stored Max Lumens value. In some embodiments, the method may proceed immediately to FIG. 12A to adjust the Target Lumens value to account for changes in the Brightness setting and/or to adjust the Scale Factor value to account for changes in brightness due to temperature changes.

In some embodiments, steps 54-62 of FIG. 6 may be repeated a number of times to minimize errors. For example, as the Scale Factor reduces, the Target Lumens value determined in step 62 reduces, which decreases the drive currents supplied to the LED chains, improves LED efficiency and changes the relative drive currents between the LED chains. When this occurs, it may be beneficial to repeat steps 54-62 to determine the chromaticity values that are expected for each LED chain at the new drive currents supplied thereto to provide a more accurate representation of the Max Lumens value. For all subsequent iterations of steps 54-62, step 90 of FIG. 11 is not performed and the previous Scale Factor value is used instead, so that the Scale Factor changes less and less with each iteration. The Max Lumens value is not updated with the Target Lumens value calculated in step 62 until all iterations are complete.

If a change in Brightness setting is detected (in step 64 of FIG. 6), the method may also proceed to FIG. 12A (in step 66). Although similar method steps are shown in FIGS. 6 and 12A, the method shown in FIG. 12A determines the Drive Currents that should be supplied to the LED chains, estimates the Total Power that should be drawn by the LED chains, and generates the Scale Factor at the present operating temperature, instead of the predetermined safe temperature used in steps 56, 58 and 60 of FIG. 6. This provides more accurate Drive Currents, Estimated Total Power and Scale Factor values for the present operating temperature. The method shown in FIG. 12A also calculates the Target Lumens using the brightness setting stored within the illumination device, and thus, provides a more accurate Target Lumens value.

In some embodiments, the method shown in FIG. 12A may begin by temporarily resetting the Scale Factor to “1” and loading the brightness setting (in step 102), for example, from the interface 20 or storage medium 23 of the illumination device. In step 104, the Target Lumens value is again calculated according to the equation:
Target Lumens=Brightness*Max Lumens*Scale Factor
this time using the brightness setting stored within the illumination device and retrieved in step 102, the Max Lumens value stored in step 62 of FIG. 6, and the Scale Factor set to “1.” Since the Scale Factor is temporarily set to “1” in step 102, the Target Lumens value calculated in step 104 may be considered a temporary Target Lumens value.

In step 106, the method determines the Actual Lumens needed from each LED chain to achieve the Target Lumens value (from step 104) at the present operating temperature. Exemplary method steps for determining the Actual Lumens needed from each LED chain are shown in FIG. 12B. While the method steps shown in FIG. 12B are similar to those shown in FIG. 7B and discussed above, there are two exceptions.

First, the x and y chromaticity values expected for each LED chain are determined (in step 114) at the present operating temperature, instead of the predetermined safe temperature, by measuring a forward voltage (Vfe_present) presently developed across each LED chain. This is achieved during operation of the illumination device by periodically turning all LED chains “off” for short periods of time (in step 108), applying a relatively small, non-operative drive current to each LED chain, one chain at a time, during the short durations of time, and measuring the forward voltage (Vfe_present) developed there across (in step 110). Methods for measuring a forward voltage are described further in co-pending application Ser. Nos. 14/314,530; 14/314,580; and 14/471,081. After the forward voltages are measured across each LED chain, the drive currents (Idrv) supplied to the LED chains to produce illumination are determined (in step 112) from the LED driver circuitry. In step 114, the x and y chromaticity values expected for each LED chain (x_i, y_i) are determined using the forward voltage (Vfe_present) measured in step 110, the drive current determined in step 112, a table of stored calibration values and one or more interpolation techniques. The x and y chromaticity values expected for each LED chain (x_i, y_i) may be determined in the same manner described above in step 74 of FIG. 7B and shown in FIGS. 8-9, except that the chromaticity values are determined for Vfe_present, instead of Vfe_safe.

As a second distinction, the method shown in FIG. 12B calculates the Actual Lumens needed from each LED chain to achieve the Target Chromaticity (xm, ym) setting and the Target Lumens (Ym) in step 116. Although Actual Lumens are calculated (in step 116 of FIG. 12B) instead of lumen proportions (in step 76 of FIG. 7B), the process is essentially the same. For example, the Target Lumens (Ym) for the combined light from four LED chains may be expressed as:
Ym=Y₁+Y₂+Y₃+Y₄
In this case, however, Ym is not set to “1,” so that Y₁, Y₂, Y₃, and Y₄ represent the Actual Lumens needed from the four LED chains to produce the Target Lumens (Ym) value determined in step 104. The Actual lumens (Y₁, Y₂, Y₃ and Y₄) may be calculated using well-known color mixing equations, the Target Chromaticity (xm, ym) values set within the illumination device, and the expected color points (x₁, y₁), (x₂, y₂), (x₃, y₃), (x₄, y₄) determined in step 114 of FIG. 12B. As these equations are well-known and readily understood by a skilled artisan, further description of such equations will be omitted herein.

In step 118, the Drive Currents (Ix) needed for each LED chain to produce the Actual Lumens at the present operating temperature are determined. According to one embodiment, the Drive Currents may be determined using the forward voltage (Vfe_present) measured for each LED chain in step 110, the Actual Lumens determined for each LED chain in step 106/116, the table of calibration values stored within the illumination device, and one or more interpolation techniques. The Drive Currents needed for each LED chain may be determined in the same manner described above in step 56 of FIG. 6 and shown in FIG. 10, except that the Drive Currents are determined at Vfe_present, instead of Vfe_safe.

In step 120, the total power (“Total Power”) drawn by all LED chains at the present operating temperature is estimated. As noted above, the power drawn by each LED chain can be estimated by multiplying a respective Drive Current determined in step 118 with a forward voltage value (Vfe_est), which is estimated for that Drive Current level at the present operating temperature. The Total Power drawn by all LED chains can then be calculated by summing the power drawn by each chain (e.g., P1+P2+P3+P4 when four LED chains are included). In one example, the forward voltage (Vfe_safe) values that were previously calibrated for each LED chain at 25° C. may be scaled (e.g., by some fixed amount or by using characterization data and a curve fitting approach) to estimate the forward voltage (Vfe_est) values corresponding to the respective Drive Currents at the present operating temperature. Alternatively, the forward voltages (Vfe_present) measured for each LED chain in step 110 may be scaled to estimate the forward voltage (Vfe_est) values corresponding to the respective Drive Currents at the present operating temperature.

In step 122, a Scale Factor is generated for adjusting the Target Lumens value to ensure that the Drive Currents determined for each LED chain (in step 118) and the estimated Total Power drawn by all LED chains (in step 120) at the present operating temperature will not exceed a maximum safe current level (“Max Current”) or a maximum safe power level (“Max Power”) attributed to the power converters (e.g., power converters 14 and 32 of FIGS. 1-2) at the present operating temperature.

An exemplary method for generating a Scale Factor for a predetermined safe temperature was described above with respect to FIG. 11. In step 122 of FIG. 12A, a Scale Factor is generated at the present operating temperature, instead of the predetermined safe temperature. While a similar method is used, additional method steps may be needed to generate the Scale Factor at the present operating temperature.

Returning to FIG. 11, the Scale Factor value is again temporarily set to “1” (in step 90). However, in this case, the present operating temperature is measured (in step 92) before the Max Power and Max Currents are determined for the power converters (in step 94). According to one embodiment, the present operating temperature can be measured by a temperature sensor (e.g., temperature sensor 28, FIG. 1), which is coupled to a circuit board or chip comprising one or more of the power converters, control circuit, driver circuitry and emission LEDs. Once the present operating temperature is measured (in step 92), the Max Current associated with each of the DC/DC converters 32 and the Max Power associated with the AC/DC converter 14 may be determined at the present operating temperature (in step 94), instead of the predetermined safe temperature.

As noted above, the Max Current may be approximately 900 mA for the white, red and green LED chains and approximately 400 mA for the blue LED chain at 25° C., and the Max Power may be approximately 16 W at 25° C. However, these values decrease significantly above the safe operating temperature. At a present operating temperature of about 75° C., for example, the Max Current of the DC/DC converters 32 and the Max Power of the AC/DC converter 14 may only be about 80% of their safe temperature (25° C.) values. Step 94 of FIG. 11 determines the Max Current and Max Power values for the power converters at the present operating temperature. According to one embodiment, the Max Current and Max Power values may be determined by linearly interpolating between the stored slope and intercept values corresponding to the I_sat vs. temperature relationships (FIG. 3) attributed to each of the power converters.

In step 96, a ratio of Max Power (from step 94) over Estimated Total Power (from step 58) is calculated for the AC/DC converter 14. In step 98, a ratio of Max Current (from step 94) over Drive Current for each LED chain (from step 56) is calculated for each of the DC/DC converters 32. The smallest of the ratios calculated in steps 96 and 98 is multiplied with the Scale Factor value (e.g., “1” from step 90 if on first iteration) and the result is stored as a new Scale Factor value (in step 100). If the result is greater than 1, the new Scale Factor value is clipped at 1.

Once the Scale Factor is generated (in step 122), the Target Lumens value is again calculated (in step 124) according to the equation:
Target Lumens=Brightness*Max Lumens*Scale Factor
using the brightness setting stored within the illumination device, the Max Lumens value calculated in step 62 of FIG. 6, and the scale factor generated in step 122 of FIG. 12A. When operating temperatures are less than or equal to the predetermined safe temperature, the method described thus far provides a precise lumen output for the particular chromaticity, white mix and brightness level settings selected for the illumination device. Above the predetermined safe temperature, the Scale Factor generated in step 122 scales the lumen output with temperature, so as not to exceed the Max Power or Max Current associated with the power converters at the present operating temperature. This avoids an “over-power” or “over-current” condition in the power converters, which improves lamp efficiency and prevents saturation of the inductive core.

In some embodiments, the drive currents supplied to the LED chains may be adjusted in step 126 (via driver circuitry 24, for example) to achieve the new Target Lumens value calculated in step 124. The illumination device may produce illumination at the new drive current levels, and the method may continue to monitor for changes in lamp settings in step 50 of FIG. 6.

In other embodiments, steps 106-124 of FIGS. 12A and 12B may be repeated a predetermined number of times to minimize errors before the drive currents are adjusted in step 126. For example, as the Scale Factor reduces with increasing temperatures above 25° C., the Target Lumens value determined in step 124 decreases, which improves LED efficiency and changes the relative drive currents between the LED chains. When this occurs, it may be beneficial to repeat steps 106-124 to determine the chromaticity values that are expected (in step 114) for each LED chain at the new drive currents to provide a more accurate representation of the Max Lumens value. For all subsequent iterations of steps 106-124, however, step 90 of FIG. 11 is not performed and the previous Scale Factor value is used instead, so that the Scale Factor changes less and less with each iteration.

In yet other embodiments, one or more of the compensation methods described in co-pending application Ser. Nos. 14/314,530; 14/314,580; and 14/471,081 may be performed to fine tune the drive currents before the adjusted drive currents are supplied to the LED chains (in step 126). The method shown in FIGS. 12A and 12B is assumed to include all such embodiments.

By performing the method steps illustrated in FIGS. 6-12B and described above, the control circuitry (e.g., control circuit 22, FIG. 1) of an illumination device is able to control the respective drive currents supplied to the emission LED chains (e.g., LED chains 26) by the driver circuits (e.g., driver circuitry 24), so as not to exceed a maximum safe power level (“Max Power”) and/or a maximum safe current level (“Max Current”) attributed to the power converters (e.g., AC/DC converter 14, DC/DC converters 32) at the present operating temperature. As noted above, the methods shown in FIGS. 6-12B are generally performed when the illumination device is first turned “on,” and any time a change in lamp settings (e.g., target chromaticity, white mix and/or brightness level) is detected during normal operation of the illumination device. However, since the Max Power and Max Current that can be safely produced by the AC/DC and DC/DC power converters are affected by changes in temperature (above the predetermined safe temperature), additional steps may be needed to fine tune the drive currents during operation of the illumination device when no changes in lamp settings are detected.

If no changes in lamp settings are detected in step 50 of FIG. 6, the method may proceed (in step 68) to the normal operation mode shown in FIG. 13A. During normal operation, in which no changes are made to the brightness level or the target chromaticity or white mix variables stored within the illumination device, the drive currents supplied to the LED chains are continually or periodically updated as the operating temperature changes over time. As drive currents increase, the operating temperature increases, which decreases the Max Current and the Max Power associated with the power converters. The method shown in FIG. 13A is used during normal operation of the illumination device to continually or periodically adjust the Scale Factor value, so as to account for temperature related changes in the Drive Current, Total Power, Max Current and/or Max Power.

In some embodiments, the method shown in FIG. 13A may begin by re-measuring the present operating temperature (in step 128). As noted above, the present operating temperature may be measured by a temperature sensor (e.g., temperature sensor 28, FIG. 1), which is coupled to a circuit board or chip comprising, e.g., one or more of the power converters, control circuit, driver circuitry and/or emission LEDs. Other means for measuring the present operating temperature may also be used.

In some embodiments, the operating temperature measured in step 128 of FIG. 13A may be compared to a previously measured operating temperature to determine if the operating temperature has changed by a certain amount. In one embodiment, a change in temperature may be detected (in optional step 130) if the operating temperature changes by about 1° C. However, the detecting step is not limited to any particular increment of temperature, may be configured to detect substantially any predetermined difference in operating temperature, and may not be performed in all embodiments.

If no change in temperature is detected (in optional step 130), the method may proceed to step 50 of FIG. 6 to continue monitoring for changes in lamp settings. If no changes in lamp settings are detected in step 50 of FIG. 6, the method may continually or periodically monitor the present operating temperature in step 128 of FIG. 13A until a change in operating temperature is detected (in step 130). If optional step 130 is not included, method steps 128-156 may be performed continually or periodically, whilst no changes in lamp settings are detected, to update the Scale Factor to account for temperature related changes.

If a change in operating temperature is detected (in optional step 130), the Actual Lumens needed from each LED chain to achieve the Target Chromaticity (xm, ym) setting stored within the illumination device and the most recently calculated Target Lumens (Ym) may be determined in step 132 for the new present operating temperature, as described above in step 106 of FIG. 12A. In step 134, the Drive Currents (Ix) needed to produce the Actual Lumens at the present operating temperature may be determined for each LED chain, as described above in step 118 of FIG. 12A. In step 136, the Drive Currents (Ix) determined in step 134 may be supplied to the LED chains via the LED driver circuitry.

In step 138, the Total Power actually drawn by all LED chains at the present operating temperature is calculated by summing the power drawn by each individual LED chain (e.g., P1+P2+P3+P4). As noted above, the power drawn by each LED chain may be calculated by multiplying the drive current presently supplied to the LED chain with a forward voltage corresponding to that drive current. In this case, however, the forward voltage values are not estimated. Instead, each forward voltage value is calculated by multiplying an input voltage supplied to a respective DC/DC converter (e.g., DC/DC converters 32 of FIG. 2) by the duty cycle of that converter. This provides a more accurate representation of the Total Power actually being drawn by all LED chains, compared to the estimates determined in steps 58 and 120.

In step 140, the Scale Factor value is updated to account for any changes in the maximum safe current level (“Max Current”) and/or the maximum safe power level (“Max Power”) of the power converter(s) at the new present operating temperature. An exemplary method for updating the Scale Factor value is shown in FIG. 13B.

Several of the method steps used in FIG. 13B to update the Scale Factor value are similar to the ones used in FIG. 11 to generate the Scale Factor value. For example, FIG. 13B may begin (in step 142) by determining the Max Power and the Max Current attributed to the power converters at the new operating temperature. The Max Power and Max Current may be determined in step 142 in the same manner as described above in step 94 of FIG. 11. In step 144, a ratio of the Max Power (from step 142) over Total Power (from step 138) is calculated for the AC/DC converter 14, similar to step 96 of FIG. 11. In step 146, a ratio of the Max Current (from step 142) over the Drive Current determined for each LED chain (in step 134) is calculated for each of the DC/DC converters 32, similar to step 98 of FIG. 11. However, the similarities between FIGS. 11 and 13B end here.

In step 148 of FIG. 13B, “1” is subtracted from the smallest of the ratios calculated in steps 144 and 146 and the result of such subtraction is used to generate a new or updated Scale Factor value. In some embodiments, the subtraction result from step 148 is added to a previously generated Scale Factor value to produce a new Scale Factor value, which is stored (in step 152). Depending on the brightness setting and the present operating temperature, the subtraction result from step 148 may be a positive value (which increases the Scale Factor value) or a negative value (which decreases the Scale Factor value).

As long as the brightness setting is small enough (e.g., roughly 50% or less), all Drive Currents determined in step 134 and the Total Power calculated in step 138 will be less than their maximum safe levels at the present operating temperature. When this occurs, the smallest of the ratios calculated in steps 144 and 146 will be some value greater than “1.” After “1” is subtracted from this value in step 148, a positive result is added to the previously generated Scale Factor to generate a new Scale Factor value, which gradually increases towards “1,” until it is clipped at 1. On the other hand, if the brightness setting and operating temperature are both high, at least one of the Drive Currents or the Total Power will exceed its maximum safe level, resulting in at least one ratio (from steps 144 or 146) that is less than “1.” When “1” is subtracted from this ratio (in step 148), a negative result is added to the previously generated Scale Factor to generate a new Scale Factor value, which gradually decreases away from “1.”

In some embodiments, the new Scale Factor value is used to calculate a new Target Lumens value (in step 154 of FIG. 13A) according to the equation provided above. As expected, increasing Scale Factor values increase the Target Lumens value, and thus, increase the drive currents supplied to the LED chains, the Total Power drawn by all LED chains, and eventually the operating temperature. Decreasing Scale Factor values have the opposite effect.

In some embodiments, the drive currents supplied to the LED chains (in step 136) may be adjusted to achieve the new Target Lumens value (in step 156). The illumination device may produce illumination at the new drive current levels, and the method may return to step 50 of FIG. 6 to monitor and detect changes in lamp settings. In other embodiments, one or more of the compensation methods described in co-pending application Ser. Nos. 14/314,530; 14/314,580; and 14/471,081 may be performed to fine tune the drive currents before the adjusted drive currents are supplied to the LED chains (in step 156). The method shown in FIG. 13A is assumed to include all such embodiments.

In some embodiments, the positive or negative subtraction result from step 148 of FIG. 14 may be scaled by a coefficient value (Ki) (in optional step 150) before the result is added to the previously generated Scale Factor to generate a new Scale Factor value (in step 152). The coefficient value (Ki) is typically much less than “1” and may be used, in some embodiments, to ensure that the control loop shown in FIG. 13 responds much faster than temperature changes. In optional step 150, the positive or negative subtraction result from step 148 is multiplied by the coefficient value (Ki) and the multiplication result is added to the previously generated Scale Factor to generate the new Scale Factor value. The new Scale Factor value may be stored (in step 152) and applied to the Target Lumens value (in step 154), as described above.

It will be appreciated to those skilled in the art having the benefit of this disclosure that this invention is believed to provide an improved illumination device and improved methods for avoiding an over-power or over-current condition in a power converter. Specifically, illumination devices and methods are provided herein for adjusting the drive currents supplied to the LED chains, so as not to exceed a maximum safe power level or a maximum safe current level attributed to one or more power converters included within the illumination device. Further modifications and alternative embodiments of various aspects of the invention will be apparent to those skilled in the art in view of this description. It is intended, therefore, that the following claims be interpreted to embrace all such modifications and changes and, accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense.

Claims

1. An illumination device, comprising:

a plurality of light emitting diode (led) chains configured to produce illumination for the illumination device;

a driver circuit coupled for generating and supplying a respective drive current to each of the plurality of led chains;

at least one power converter coupled for supplying power to the driver circuit, wherein the at least one power converter comprises a maximum safe current level or a maximum safe power level, which varies with temperature;

a temperature sensor coupled for measuring a present temperature associated with the at least one power converter; and

a control circuit coupled to the temperature sensor and the driver circuit, wherein the control circuit is configured for:

determining the maximum safe current level or the maximum safe power level of the at least one power converter at the present temperature; and

adjusting the respective drive currents supplied to the plurality of led chains, so as not to exceed the maximum safe current level or the maximum safe power level at the present temperature;

an interface coupled for receiving a chromaticity setting, and wherein the control circuit is further configured for determining a maximum lumens value that can be safely produced by all led chains at a predetermined safe temperature to achieve the chromaticity setting; and

wherein the interface is further coupled for receiving a brightness setting, and wherein the control circuit is further configured for determining a target lumens value that can be safely produced by all led chains at the present temperature to achieve the chromaticity setting without exceeding the maximum safe current level or the maximum safe power level associated with the at least one power converter.

2. The illumination device as recited in claim 1, wherein the control circuit is configured to determine the maximum lumens value by:

determining, for each led chain, a lumen proportion needed from each led chain to achieve the chromaticity setting at the predetermined safe temperature;

determining, for each led chain, a relative lumens needed from the led chain to achieve the lumen proportion determined for the led chain, assuming only one of the plurality of led chains is driven with a maximum drive current;

calculating, for each led chain, a ratio of the relative lumens determined for the led chain over a maximum lumen output for the led chain;

determining, for each led chain, an actual lumens needed from the led chain to achieve the chromaticity setting at the predetermined safe temperature by dividing the relative lumens needed from the led chain by a largest of the calculated ratios; and

summing the actual lumens needed from each led chain to determine the maximum lumens value that can be produced by all led chains at the predetermined safe temperature to achieve the chromaticity setting.

3. The illumination device as recited in claim 2, wherein the control circuit is configured to determine the lumen proportions needed from each led chain to achieve the chromaticity setting at the predetermined safe temperature by:

determining, for each led chain, chromaticity values that are expected for the led chain using a forward voltage calibrated for the led chain at the predetermined safe temperature, the respective drive current supplied to the led chain, a table of stored calibration values correlating forward voltage and drive current to chromaticity at a plurality of different temperatures, and one or more interpolation techniques; and

calculating the lumen proportions needed from each led chain to achieve the chromaticity setting at the predetermined safe temperature using the expected chromaticity values.

4. The illumination device as recited in claim 2, wherein the at least one power converter comprises a first power converter coupled for supplying a DC voltage to a plurality of second power converters, each of which are coupled for producing a forward voltage on a respective one of the led chains, and wherein the maximum safe power level of the first power converter and the maximum safe current levels of the second power converters varies with temperature above the predetermined safe temperature.

5. The illumination device as recited in claim 4, wherein the control circuit is further configured to determine the maximum lumens value by:

determining, for each led chain, a drive current needed to produce the actual lumens needed from the led chain to achieve the chromaticity setting at the predetermined safe temperature;

estimating a total power drawn by all led chains combined at the predetermined safe temperature;

determining the maximum safe power level and the maximum safe current level of the power converters at the predetermined safe temperature;

calculating a ratio of the maximum safe power level at the predetermined safe temperature over the total power estimated at the predetermined safe temperature;

calculating, for each led chain, a ratio of the maximum safe current level at the predetermined safe temperature over the drive current determined for the led chain at the predetermined safe temperature;

using a smallest of the calculated ratios to generate a scale factor; and

applying the scale factor to the maximum lumens value.

6. The illumination device as recited in claim 1, wherein the control circuit is configured to determine the target lumens value by:

applying the brightness setting to the maximum lumens value to generate a temporary target lumens value;

determining, for each led chain, an actual lumens needed from the led chain to achieve the temporary target lumens value at the present temperature;

determining, for each led chain, a drive current needed to produce the actual lumens at the present temperature;

estimating a total power drawn by all led chains combined at the present temperature;

calculating a ratio of the maximum safe power level determined at the present temperature over the total power estimated at the present temperature;

calculating, for each led chain, a ratio of the maximum safe current level determined at the present temperature over the drive current determined for the led chain at the present temperature;

using a smallest of the calculated ratios to generate a scale factor; and

calculating the target lumens value by applying the scale factor and the brightness setting to the maximum lumens value.

7. The illumination device as recited in claim 6, wherein the control circuit is configured to determine the actual lumens needed from each led chain to achieve the temporary target lumens value at the present temperature by:

periodically turning the plurality of led chains off for short durations of time;

measuring a forward voltage presently developed across each led chain by applying a non-operative drive current to each led chain, one chain at a time, during the short durations of time the plurality of led chains are periodically turned off;

determining chromaticity values that are expected for each led chain using the forward voltage measured across each led chain, the respective drive current supplied to each led chain, a table of stored calibration values correlating forward voltage and drive current to chromaticity at a plurality of different temperatures, and one or more interpolation techniques; and

calculating the actual lumens needed from each led chain to achieve the temporary target lumens value using the expected chromaticity values and the chromaticity setting.

8. The illumination device as recited in claim 6, wherein the control circuit is configured for adjusting the respective drive currents supplied to the plurality of led chains, so as to achieve the target lumens value.

9. The illumination device as recited in claim 6, wherein the control circuit is configured for periodically readjusting the respective drive currents supplied to the plurality of led chains to account for changes in the present temperature by:

measuring a new present temperature;

determining, for each led chain, an actual lumens needed from the led chain to achieve the chromaticity setting and the target lumens value at the new present temperature;

determining, for each led chain, a drive current needed to produce the actual lumens at the new present temperature;

applying the determined drive currents to the led chains;

determining a total power drawn by all led chains combined at the new present temperature;

updating the scale factor to account for changes in the maximum safe power level and/or the maximum safe current level of the power converters at the new present temperature;

recalculating the target lumens value using the updated scale factor; and

adjusting the respective drive currents supplied to the plurality of led chains, so as to achieve the recalculated target lumens value.

10. The illumination device as recited in claim 9, wherein the control circuit is configured for updating the scale factor by:

determining the maximum safe power level and the maximum safe current level of the power converters at the new present temperature;

calculating a ratio of the maximum safe power level at the new present temperature over the total power determined at the new present temperature;

calculating, for each led chain, a ratio of the maximum safe current level at the new present temperature over the drive current determined for the led chain;

subtracting 1 from a smallest of the calculated ratios to generate a subtraction result; and

adding the subtraction result to the scale factor to update the scale factor.

11. The illumination device as recited in claim 9, wherein the control circuit is configured for updating the scale factor by:

determining the maximum safe power level and the maximum safe current level of the power converters at the new present temperature;

calculating a ratio of the maximum safe power level at the new present temperature over the total power determined at the new present temperature;

calculating, for each led chain, a ratio of the maximum safe current level at the new present temperature over the drive current determined for the led chain;

subtracting 1 from a smallest of the calculated ratios to generate a subtraction result;

multiplying the subtraction result with a coefficient value to generate a multiplication result; and

adding the multiplication result to the scale factor to update the scale factor.

12. A method for controlling a lumen output of an illumination device comprising a plurality of light emitting diode (led) chains and a plurality of power converters, so as not to exceed a maximum safe power level and a maximum safe current level of the power converters at a present temperature, the method comprising:

measuring a the present temperature associated with the at least one power converter;

determining the maximum safe current level and the maximum safe power level of the power converters at the present temperature; and

adjusting drive currents supplied to each of the plurality of led chains, so as not to exceed the maximum safe current level and the maximum safe power level at the present temperature;

receiving a chromaticity setting and a brightness setting for the illumination device;

determining a maximum lumens value that can be safely produced by all led chains at a predetermined safe temperature to achieve the chromaticity setting; and

determining a target lumens value that can be safely produced by all led chains at the present temperature to achieve the chromaticity setting without exceeding the maximum safe current level or the maximum safe power level associated with the power converters.

13. The method as recited in claim 12, wherein said determining a maximum lumens value comprises:

determining, for each led chain, a lumen proportion needed from each led chain to achieve the chromaticity setting at the predetermined safe temperature;

determining, for each led chain, a relative lumens needed from the led chain to achieve the lumen proportion determined for the led chain, assuming only one of the plurality of led chains is driven with a maximum drive current;

calculating, for each led chain, a ratio of the relative lumens determined for the led chain over a maximum lumen output for the led chain;

determining, for each led chain, an actual lumens needed from the led chain to achieve the chromaticity setting at the predetermined safe temperature by dividing the relative lumens needed from the led chain by a largest of the calculated ratios; and

summing the actual lumens needed from each led chain to determine the maximum lumens value that can be safely produced by all led chains at the predetermined safe temperature to achieve the chromaticity setting.

14. The method as recited in claim 13, wherein said determining the lumen proportions needed from each led chain to achieve the chromaticity setting at the predetermined safe temperature comprises:

determining, for each led chain, chromaticity values that are expected for the led chain using a forward voltage calibrated for the led chain at the predetermined safe temperature, the drive current supplied to the led chain, a table of stored calibration values correlating forward voltage and drive current to chromaticity at a plurality of different temperatures, and one or more interpolation techniques; and

calculating the lumen proportions needed from each led chain to achieve the chromaticity setting at the predetermined safe temperature using the expected chromaticity values.

15. The method as recited in claim 13, wherein said determining a maximum lumens value further comprises:

determining, for each led chain, a drive current needed to produce the actual lumens needed from the led chain to achieve the chromaticity setting at the predetermined safe temperature;

estimating a total power drawn by all led chains combined at the predetermined safe temperature;

determining the maximum safe power level and the maximum safe current level of the power converters at the predetermined safe temperature;

calculating a ratio of the maximum safe power level at the predetermined safe temperature over the estimated total power;

calculating, for each led chain, a ratio of the maximum safe current level at the predetermined safe temperature over the drive current determined for the led chain;

using a smallest of the calculated ratios to generate a scale factor; and

applying the scale factor to the maximum lumens value.

16. The method as recited in claim 12, wherein said determining a target lumens value comprises:

applying the brightness setting to the maximum lumens value to generate a temporary target lumens value;

determining, for each led chain, an actual lumens needed from the led chain to achieve the temporary target lumens value at the present temperature;

determining, for each led chain, a drive current needed to produce the actual lumens at the present temperature;

estimating a total power drawn by all led chains combined at the present temperature;

calculating a ratio of the maximum safe power level determined at the present temperature over the total power estimated at the present temperature;

calculating, for each led chain, a ratio of the maximum safe current level determined at the present temperature over the drive current determined for the led chain at the present temperature;

using a smallest of the calculated ratios to generate a scale factor; and

calculating the target lumens value by applying the scale factor and the brightness setting to the maximum lumens value.

17. The method as recited in claim 16, wherein said determining the actual lumens needed from each led chain to achieve the temporary target lumens value at the present temperature comprises:

periodically turning the plurality of led chains off for short durations of time;

measuring a forward voltage presently developed across each led chain by applying a non-operative drive current to each led chain, one chain at a time, during the short durations of time the plurality of led chains are periodically turned off;

determining chromaticity values that are expected for each led chain using the forward voltage measured across each led chain, the drive current supplied to each led chain, a table of stored calibration values correlating forward voltage and drive current to chromaticity at a plurality of different temperatures, and one or more interpolation techniques; and

calculating the actual lumens needed from each led chain to achieve the temporary target lumens value using the expected chromaticity values and the chromaticity setting.

18. The method as recited in claim 16, further comprising adjusting the drive currents supplied to the plurality of led chains, so as to achieve the target lumens value.

19. The method as recited in claim 16, further comprising periodically readjusting the drive currents supplied to the plurality of led chains to account for changes in the present temperature by:

measuring a new present temperature;

determining, for each led chain, an actual lumens needed from the led chain to achieve the chromaticity setting and the target lumens value at the new present temperature;

determining, for each led chain, a drive current needed to produce the actual lumens at the new present temperature;

applying the determined drive currents to the led chains;

determining a total power drawn by all led chains combined at the new present temperature;

updating the scale factor to account for changes in the maximum safe power level and/or the maximum safe current level of the power converters at the new present temperature;

recalculating the target lumens value using the updated scale factor; and

adjusting the drive currents supplied to the plurality of led chains, so as to achieve the recalculated target lumens value.

20. The method as recited in claim 19, wherein said updating the scale factor comprises:

determining the maximum safe power level and the maximum safe current level of the power converters at the new present temperature;

calculating a ratio of the maximum safe power level determined at the new present temperature over the total power determined at the new present temperature;

calculating, for each led chain, a ratio of the maximum safe current level determined at the new present temperature over the drive current determined for the led chain at the new present temperature;

subtracting 1 from a smallest of the calculated ratios to generate a subtraction result; and

adding the subtraction result to the scale factor to update the scale factor.

21. The method as recited in claim 19, wherein said updating the scale factor comprises:

determining the maximum safe power level and the maximum safe current level of the power converters at the new present temperature;

calculating a ratio of the maximum safe power level determined at the new present temperature over the total power determined at the new present temperature;

calculating, for each led chain, a ratio of the maximum safe current level determined at the new present temperature over the drive current determined for the led chain at the new present temperature;

subtracting 1 from a smallest of the calculated ratios to generate a subtraction result;

multiplying the subtraction result with a coefficient value to generate a multiplication result; and

adding the multiplication result to the scale factor to update the scale factor.

22. A controller for an illumination device, the controller comprising:

a control circuit coupled to a light-emitting diode (led) driver circuit, wherein the control circuit is configured for determining a present temperature of the illumination device, and adjusting respective drive currents supplied by the led driver circuit to a plurality of led chains, so as not to exceed a maximum safe current level or a maximum safe power level of a power supply providing power to the led driver circuit at the present temperature; and

an interface coupled to the control circuit and coupled for receiving a chromaticity setting;

wherein the control circuit is further configured for determining a maximum lumens value that can be safely produced by the plurality of led chains at a predetermined safe temperature to achieve the chromaticity setting; and

wherein the interface is further coupled for receiving a brightness setting, and the control circuit is further configured for determining a target lumens value that can be safely produced by the plurality of led chains at the present temperature to achieve the chromaticity setting without exceeding the maximum safe current level or the maximum safe power level of the power supply.

23. A method for controlling a lumen output of an illumination device, the method comprising

measuring, by temperature sensing circuitry, a present temperature of the illumination device;

adjusting, by control circuitry communicatively coupled to the temperature sensing circuitry, drive currents supplied to each of a plurality of light-emitting diode (led) chains, so as not to exceed a maximum safe current level or a maximum safe power level of a power supply operatively coupled to the plurality of led chains, at the measured present temperature;

receiving, via interface circuitry communicatively coupled to the control circuitry, a target chromaticity setting and a target brightness setting for the illumination device;

determining a maximum lumens value that can be safely produced by the plurality of led chains at a predetermined safe temperature to achieve the target chromaticity setting; and

determining a target lumens value that can be safely produced by the plurality of led chains at the measured present temperature to achieve the target chromaticity setting without exceeding the maximum safe current level or the maximum safe power level of the power supply.

24. A controller for an illumination device, the controller comprising:

a control circuit coupled to one or more light-emitting diode (led) driver circuits, the control circuit configured to:

measure, via temperature sensing circuitry communicatively coupled to the control circuit, a present temperature of the illumination device;

determine a scale factor to be applied to the respective drive currents of each of a plurality of led chains to achieve a target chromaticity setting for the illumination device without exceeding a maximum current level of the one or more led driver circuits at the measured present temperature;

determine a maximum lumens value that can be produced by the plurality of led chains to achieve the target chromaticity setting for the illumination device without exceeding the maximum current level of the one or more led driver circuits at the measured present temperature;

determine, based on the scale factor, the maximum lumens value, and a target brightness setting, a target lumens value that can be produced by the plurality of led chains to achieve the target chromaticity setting for the illumination device without exceeding the maximum current level of the one or more led driver circuits at the measured present temperature;

update the target lumens value in response to a change in the target chromaticity setting, the brightness setting, or the measured present temperature; and

adjust, based on the scale factor and the maximum lumens value, the respective drive currents supplied to each of the plurality of led chains to achieve the target lumens value; and

an interface communicatively coupled to the control circuit to receive the target chromaticity setting or the target brightness setting.

25. The controller of claim 24, wherein the interface comprises at least one of a wireless interface, a wired interface, or a user interface.